1. Field of the Invention
The present invention relates to methods of chroma upscaling, and particularly to a method of setting a chroma of a pixel according to a luma of a reference pixel, which does not require a division operation to set a target pixel's chroma.
2. Description of the Prior Art
Signal processing technology improves daily, and customer requirements for video quality increase daily. However, in traditional video technology, because signal processing speed is relatively slow, and transmission bandwidth is limited, most technologies adopt a data format in which every pixel contains luma information, but only pixels in every other row contain chroma information. In order to satisfy customer requirements for video quality, media players in the current market typically perform a chroma upsampling process on the video data formatted as mentioned above before playing.
Typical chroma upscaling techniques employ a finite impulse response (FIR) method to interpolate pixel chroma information. Please refer to FIG. 1. An image frame 10 shown in FIG. 1 comprises three neighboring pixels 11, 12, 13 located in a same row. In raw video data, each of the three pixels 11, 12, 13 comprises respective luma information Y1, Y2, Y3. However, only the pixel 11 and the pixel 13 comprise respective chroma information C1, C3. The pixel 12 does not comprise chroma information. In FIG. 1, the pixel 11 and the pixel 13 are filled with diagonal lines to show that they comprise chroma information. Taking the pixel 12 as a target pixel for chroma upscaling, and taking the pixel 11 and the pixel 13 as reference pixels, when performing a second order FIR interpolation method, a chroma C21 of the target pixel 12 is based on the respective chroma information C1, C3 of the reference pixel 11 and the reference pixel 13, and set by the following formula:C21=(C1+C3+1)/2
The chroma C21 represents the chroma of the pixel 12, which is determined by the chroma upscaling method of the prior art.
Please refer to FIG. 2. FIG. 2 is a graph showing distribution of the luma information Y1, Y2, Y3 and the chroma information C1, C21, C3 for the three pixels in FIG. 1 following the above chroma upscaling method. In general, for video data, a trend in chroma change between neighboring pixels is very similar to a trend in luma change between the neighboring pixels. However, FIG. 2 clearly shows that, where the luma Y2 and the luma Y3 are equal, after chroma upscaling, the interpolated chroma C21 is lower than the chroma C3, which is a different trend from the lumas Y1, Y2, Y3. Therefore, the prior art method described above for chroma upscaling of video data often produces a chroma change trend that does not match the respective luma change trend, causing sub-optimal video quality.
Because in video data, the trend in chroma change between neighboring pixels typically matches the relative trend in luma change of the neighboring pixels, the prior art provides a chroma upscaling method which sets a weight of the reference pixel chroma based on a luma gradient of the reference pixel, then sets the target pixel chroma based on the reference pixel chroma and weight, thus completing chroma upscaling. This can be seen in U.S. Pat. No. 6,297,801, where Hong Jiang's “Edge-Adaptive Chroma Up-Conversion” describes this chroma upscaling method. However, because the trend in luma change between the reference pixel and the target pixel is used to set the reference pixel chroma and weight, a high number of division operations are required, which requires a calculations burden that is not worth implementing in video hardware.